For high fidelity (Hi-Fi) audio reproduction from digital sources such as audio compact disc (CD), super-audio CD (SACD), DVD-audio (DVDA), and digital audio broadcasts (DAB) using digital audio reproduction equipment, the measured root-means-square (rms) values of analog signals provided by the digital audio reproduction equipment are typically approximately 2 volts. Digital-to-analog converters (DACs) are normally installed in the digital audio reproduction equipment for converting digital audio data retrieved from the digital sources into analog signals. The analog signals provided by modern DACs are either current-based or voltage-based. If the DACs provide current-based analog signals, then conversion of the current-based analog signals into the corresponding voltage equivalent is required before the analog signals are to be further provided to subsequent stage amplifiers.
A commonly used technique employs a circuit built with operational amplifiers (op-amps) thereon for such current-to-voltage conversion. As known to ordinary persons skilled in the art, the circuit is usually termed trans-impedance amplifier or current-to-voltage (I/V) amplifier. Conventionally, the circuit is configured such that the analog signals provided by the DAC bias the inverting input of the op-amp of the circuit and a voltage output is developed across a feedback resistor that is connected between the output of the op-amp to the inverting input of the op-amp. In such a configuration, the output of the DAC operates with respect to a virtual ground and the non-inverting input of the op-amp operates with respect to a reference ground. In addition, the circuit also operates with enabled negative feedback.
In general, for excellent Hi-Fi audio reproduction, circuits operating without negative feedback are highly desired to prevent the quality deterioration of the reproduced audio due to negative feedback. In addition, the circuits preferably operate in conformance with the operating characteristics of Class-A amplifiers for achieving low output impedance. Furthermore, operating the circuits to conform to performances of Class-A amplifiers is also essential for ensuring proper operation of the circuits regardless of the voltage characteristics of the power supply being provided to the circuits. Lastly, the absence of feedback loop circulating within the circuits is also desired. Regrettably, existing circuits are however unable to meet at least one of the foregoing criterion required for excellent audio reproduction.